As is well known, magnetic tape is a medium often used to provide input and receive output information from a computer. This is accomplished by use of a tape-transport system where the magnetic tape is transported between a supply reel and a take-up reel. These reels are of relatively high mass and inertia. Therefore, the tape-transport system utilizes a capstan to actually move the tape past a read-write head at writing and reading speeds. Just as importantly, the capstan which is low mass and inertia relative to the tape reels, is used to bring the tape up to read-write speed from a stopped condition or from read-write speed to stop. It is also used to reverse tape direction which normally entails bringing the tape to a stopped condition and driving it up to read-write speed in the opposite direction. The capstan is also used to move the tape at high speed, e.g., during rewind.
In most present-day, tape-transport systems the capstan is driven by its own motor, the precise control of which is critical to the efficient operation of the tape-transport system.
Precise control of the motor-driven capstan is necessary since it is important to waste as little tape as possible during the interval that the tape is being brought from stop to read-write speed, from read-write speed to stop or during a reverse tape direction interval when reading and/or writing does not occur. Thus, these intervals should occupy as little time and tape length as possible without sacrifice of accuracy of tape formatting or stability of the system.
Precise control of the motor-driven capstan is also important during speed up to read-write speed so that the speed of the tape will not overshoot tape read-write speed with the result of a period of instability during which the capstan motor hunts the normal running speed of the tape which further expands the interval during which reading or writing cannot take place or provides distortion in the data if reading or writing is permitted to occur.
As briefly mentioned above, accurate control of the motor-driven capstan is important to achieve precise tape formatting. In other words, accurate control of speed-up and speed-down intervals of the tape assures that all such intervals during which tape speed is changing fall within the inter-data gap, i.e., between data blocks on the tape. Precise control permits the intervals of tape-speed changes to be short with the result that the inter-data gaps can be correspondingly short. This, along with elimination of overshoot when tape speed reaches read-write speed, lessens the amount of tape used for inter-data gaps.
State-of-the-art tape-transport systems utilize ramp or sawtooth voltages to drive the capstan motor during speed-changing conditions. These ramp voltages may be combined with a voltage proportional to the velocity of the capstan motor before application to the motor-drive elements. However, accuracy of capstan motor control is determined by the accuracy of control of ramp-voltage generation.
The ramp voltage in present-day systems is generated by analog-integration techniques which, of course, implies the use of R-C networks to generate the sawtooth. As is well known, analog-integrator circuits are subject to drift and instability due to many factors such as temperature changes. Since intervals during which tape is brought from stop to running speed or from running speed to a stopped condition are ideally kept constant, use of analog circuitry with its inherent drift and instability presents a problem in which the time interval of the ramp voltage may vary. Such time-interval variation may be compensated by use of sophisticated circuitry by which the slope of the ramp voltage may be adjusted to cause the ramp voltage time interval to remain more or less constant. This, of course, is a trade-off and itself may exacerbate the running-speed overshoot problem already inherent in analog techniques.
The combination of the present invention contemplates the use of digital techniques to provide the ramp voltage instead of the analog integrator. Use of the digital arrangement of the present invention virtually eliminates the problems associated with conventional circuitry. Thus, the digital method of the present invention provides a ramp voltage whose time interval is kept constant without the necessity of varying the slope of the ramp. Further, with the use of the present invention, the speed of the capstan is brought exactly to desired running speed without overshoot.
Thus, the combination of present invention provides a capstan-motor control wherein the problems of drift and instability associated with present-day systems are eliminated. In addition, the present invention uses fewer components and provides a better system for a lower cost.
Specifically, the present invention contemplates a capstan-motor control in which a motor which drives the capstan provides a feedback voltage which is proportional to the instantaneous velocity of the motor to a summing junction by means of a tachometer generator connected to the motor. The summing junction also receives the ramp voltage provided by the digital circuitry. The motor which is connected to the summing junction is driven by any difference in voltage between the ramp and the tachometer voltage. The ramp voltage is provided essentially by a counter which counts up or down until it matches desired speed represented by a digitally-coded (e.g., binary) input to a digital comparator which also receives the counter's digitally-coded sum as an input. The output of the counter is continously converted to an analog voltage before application to the summing junction. The counter counts up or down in synchronism with a clock-pulse source and provides a serrated-shaped or stepped ramp in which steps occur in synchronism with the period of the clock pulses. When the motor has attained the desired running speed or a desired stopped condition, the ramp voltage levels out to a constant or a zero voltage, respectively.